1. Field of the Invention
This invention relates generally to catalytic converters in exhaust systems for automobiles and other vehicles and equipment with internal combustion engines and, particularly, to systems for maintaining desired temperature levels and reducing "cold start" emissions in such catalytic converters.
2. State of the Prior Art
Most vehicle exhaust systems and particularly exhaust systems of vehicles powered by gasoline-fueled internal combustion engines are equipped with catalytic converters for reducing noxious emissions in exhaust gases. The most effective current technology catalytic converters comprise ceramic substrates coated with one or more noble metal catalysts, such as platinum, palladium, or rhodium. The preferred noble metal for high temperature hydrocarbon reduction is palladium, and rhodium is effective for improving nitrous oxide and carbon monoxide emissions. So called 3-way catalytic converters typically include combinations of these noble metals that catalyze two oxidation reactions that oxidize carbon monoxide to carbon dioxide and oxidize hydrocarbons to carbon dioxide and water, and, at the same time, that reduce nitrogen oxides to nitrogen and oxygen. These reactions are very effective at certain high temperatures. However, until the catalyst is warmed up to its light-off temperature, defined as the temperature required to oxidize 50% of the hydrocarbons, the effectiveness of catalytic converters is very low. For example, J. C. Summers et al., in their paper "Use of Light-Off Catalysts to Meet the California LEV/ULEV Standards," Catalysts and Emission Technology, Society of Automotive Engineers Special Publication No. 968, Warrendale, Pa., 1993, reported that roughly 60-80% of the tailpipe hydrocarbon emissions occur during the initial cold start phase.
To reach light-off temperature more quickly, it is desirable to retain exhaust heat as much as possible in the catalytic converter, at least until the light-off temperature, usually in the range of about 600.degree.-800.degree. F. (250.degree.-350.degree. C.), is reached. Providing an insulation jacket around the catalytic converter can help to retain heat. However, the temperature of a catalytic converter during extended operation, once the light-off temperature is reached, can continue to rise very rapidly from the exothermic heat of the catalytic reactions with the exhaust gases. If the heat generated during extended operation or from fuel-rich gases reacted in the catalytic converter cannot be dissipated efficiently, it can build up to a point that accelerated aging of the catalyst or even permanent damage to the catalytic converter or to adjacent components or objects can result. Therefore, the maximum desired operating range for a catalytic converter is usually about 1,500.degree. F. (815.degree. C.). Therefore, simply wrapping a catalytic converter with insulation to retain heat for light-off temperature would also have the detrimental effect of retaining too much heat at high temperature levels when the catalytic converter should be dissipating heat to avoid damage.
The U.S. Pat. No. 5,163,289, issued to D. Bainbridge, discloses an insulation jacket around a catalytic converter wherein the insulation is a refractory fiber that conducts heat better at higher temperatures than at lower temperatures, which at least recognized the problem and was a start in a helpful direction. Further, improvements in maintaining heat above light-off temperature for longer times during non-operation periods with a combination of vacuum insulation and phase-change thermal storage material while dissipating heat in high-temperature operation periods with a combination of variable conductance insulation and/or metal-to-metal thermal shunt mechanisms are shown in U.S. Pat. No. 5,477,676, issued to Benson et al. The Benson et al. system disclosed in that U.S. Pat. No. 5,477,676 is effective to store heat and maintain light-off temperatures longer and thereby to reduce "cold start" emissions when it is still hot, such as after short stops of several hours or even several days. Unfortunately, however, it actually exacerbates the "cold start" problem after longer stops when the entire catalytic converter system, including the phase-change thermal storage material, has had time to cool below light-off temperature. In the latter circumstances, upon restarting, not only must the engine exhaust gas reheat the catalyst substrate, it must also reheat the phase-change thermal storage material at the same time to light-off temperature and above, which actually prolongs the "cold start" emissions for substantially longer times than if there was no such heat storage material in the catalytic converter.
Further, even if the heat retention in the catalytic converter during stops could be extended for even longer times by more or better insulation to further minimize the frequency of periods when the temperature does actually fall below light-off temperature, there will still be some occasions for most vehicles when they sit unused long enough for the temperatures of their catalytic converters to fall below light-off temperature. The ensuing periods of high "cold start" emissions are undesirable and may disqualify vehicles equipped with even such improved catalytic converters as those shown in the Benson et al. patent from "ultra-low emissions vehicle" (ULEV) classification. Also, while the electrically heated and controlled hydrogen source getter used in the Benson et al. patent to disable its vacuum insulation and thereby prevent the catalytic converter from overheating is effective for that purpose, it requires complex and expensive wiring, sensors, and control circuits. Such wiring, sensors, and control circuits make the catalytic converter and the vehicle exhaust system vulnerable to failures and increases likelihood of needing repairs and replacements of such equipment with concomitant increases of risks for warranty liabilities.
Therefore, while the Bainbridge and Benson et al. patents and other developments represent significant advancements in "cold start" emissions control with catalytic converters, remaining problems of exacerbated or prolonged "cold start" emissions after circumstances of eventual cool-down below light-off temperature and of active electric wiring, sensors, and control system failures still needed resolutions.